1. Field of the Invention
The invention relates to liquid purification and to filters employing granular filter media, with self-graded media during upwash of the filter. A perforated diaphragm hold-down permits high capacity upflow filtration, downflow filtration, and combined up and down flow filtration. The media is selected for settling velocities that allows desired gradation to be achieved, often with a coarse layer at the exit side of the media to permit screen hold-down without causing the screen to be the finest filter layer.
2. Description of the Prior Art
Sand filters are well known as filtering devices for water and many other liquids. Several configurations of sand filters are well known, including down-flow filters and up-flow filters.
Down-flow filters represent the simplist form, consisting of a tank containing a filtering medium such as sand and having a top inlet for pressurized water and a bottom outlet for filtered water. Where the filtering medium is sand, the sand will be of various sizes. To remove the accumulated dirt from the sand, water is pumped up through the sand at a velocity sufficient to lift the sand and roll it around. At the conclusion of washing, the sand settles naturally and, accordingly, is graded in the filter with the fastest settling particles at the bottom and the slowest settling particles at the top. If all particles are of substantially the same material, the largest particles are at the bottom of the filter and the smallest are at the top. When the filtering operation is taking place, the finest particles in the liquid are filtered out by immediate contact with the finest particles of said at the top of the filter, and the remaining solids merely build up on the top of the filter. Very little material is caught in the bed of sand.
Typical operating conditions for a standard down-flow sand filter are flow rates of 1/2 to 11/2 gpm/sq. ft. of cross section and a dirt holding capacity of 1/2 to 1 lb.sq. ft. of cross section. Most of the pressure drop is due to passage across the thin layer of filtered solids in the upper bed of the filter.
Representative art teaching down-flow filters includes U.S. Pat. Nos. 184,024 to Stewart, 178,972 to Stewart, and 454,340 to Fulton.
The disadvantages of down-flow filters have suggested that an up-flow filter is a far more practical filtering device, but up-flow filters present a number of problems. The greater efficiency of an up-flow filter is attributed to the same classification of particles that occurs in the washing cycle of down-flow filters: the largest particles of sand settle to the bottom of the filter and the smallest at the top. Then, as the liquid to be filtered is pumped into the sand from the bottom, the coarser impurities are trapped in the coarser layer of sand and the finer impurities pass into finer layers of sand before being trapped.
Typical operating conditions for the standard up-flow sand filter are flow rates of 8-12 gpm/sq. ft. cross section and dirt holding capacities of up to 10 lb./sq. ft. cross section. The filterable solids are distributed throughout the sand and the pressure drop is likewise distributed across the entire body of sand. Thus, if the sand can be held in place, the flow rates can be much higher than in a down-flow filter for the same pressure drop.
A number of attempts in the prior art have faced the problem of holding the sand in place during high filtering rates in an up-flow filter. U.S. Pat. No. 620,621 to Veazie teaches containing the sand layer between an upper and lower foraminous diaphragm, with the upper diaphragm being connected to a device for applying pressure to sand layers to hold the sand particles close together for efficient filtering. U.S. Pat. No. 3,278,031 to Rosaen teaches a piston arrangement for compressing the filtering medium under a perforated plate, and the pressure of the piston is released to allow medium to expand for washing. U.S. Pat. No. 2,723,761 to Van Der Made et. al. teaches an up-flow filter that routes some of the liquid to the top of the filter to supply compacting pressure to the bed of filtering media. U.S. Pat. No. 3,202,286 to Smit teaches the use of an open grate across the top of the filtering media to hold the media in place through natural bridging between elements of the grate.
A problem found in prior art up-flow filters is that the filter rate is limited by the need to hold the top layers of the filtering media in place. The foraminous devices used on the upper layer of the media in the Veazie, Rosaen, and Smit patents will yield some of the filter material through the perforations of the hold-down device if the flow rate is high enough. Alternatively, if the hold-down device is pressurized to oppose high filter rates, as in the Van der Made et. al. patent, there is danger that the pressurized water will channel to the filtrate outlet, and all filtering action will immediately cease. If a mechanical hold-down device is created with small enough perforations to physically retain the filter media against high pressure without bridging of the media particles, the hold-down device itself may become the finest layer of filtering media and be subject to rapid clogging, or the upper layer of media may be required to be coarse enough that extremely thorough filter action cannot be achieved.
A further problem exists in cleaning an up-flow filter. It is desirable to loosen the beds of the filter media and suspend each particle so that it can move freely, thereby releasing dirt trapped in the interstices. To accomplish this task in apparatus such as that taught in the Smit patent, it is necessary to use wash flow rates greater than filter flow rates, often requiring a separate wash pump for the task and often requiring air in the wash liquid to lift the media. Furthermore, in all prior up-flow up-wash graded filters, the wash rate is limited according to the rate that will lift the finest particles of the filter media completely out of the filter housing. During an up-wash process, the finest media particles form a visible plane at the top of the wash flow, and the adjustment of flow rates between a rate that will retain the particles and a rate that will wash the particles away must be carfully controlled.
Ideally, the filter media should be cheap and readily available, with sand being a preferred material. In the up-wash process, it is known that the media will be self-sorting into layers graded according to the size of the particles when all particles are of similar density, the larger particles settling at the bottom of the filter and the smaller particles at the top. If in rinsing the filter the finest particles are to be retained and not blown out the top of the filter with the dirt, then there is an additional limitation that the largest particles must be of sufficiently small size that they can be lifted and suspended in the wash water while the wash flow rate is maintained below the rate that will remove the finest particles from the top of the filter. Ordinarily, the settling rates of various sized particles of similar density are such that maximum size ratio between the largest and smallest media particles is approximately 3:1, which is not a very great range.
The present invention seeks to solve these and other problems of the prior art, as will be disclosed below.